The invention relates generally to semiconductor devices and integrated circuit fabrication and, in particular, to fabrication methods and device structures for bipolar junction transistors and heterojunction bipolar transistors.
Bipolar junction transistors may be found, among other end uses, in radiofrequency transceivers, multi-gigabit analog-to-digital converters, optical communication, automotive radar, and high-speed circuits. Bipolar junction transistors may be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the favorable characteristics of both transistor types on the same chip.
Bipolar junction transistors are three-terminal electronic devices that include an emitter, an intrinsic base, and a collector arranged such that the intrinsic base is situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased. The collector-emitter current may be controlled by the base-emitter voltage.
A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor in which two or more of the emitter, intrinsic base, and/or collector are composed of semiconductor materials with unequal band gaps, which creates heterojunctions instead of homojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be composed of silicon, and the base of a heterojunction bipolar transistor may be composed of silicon germanium (SiGe), which is characterized by a narrower band gap than silicon. In wireless designs for radiofrequency applications, the interfaces between the antenna and the electronic circuits are provided by a low-noise amplifier (LNA) and a power amplifier (PA), each of which may include a heterojunction bipolar transistor with a SiGe base.
Improved fabrication methods and device structures are needed for bipolar junction transistors and heterojunction bipolar transistors.